Unused power exists in various forms such as industrial machines, human activity, vehicles, structures and environment sources. Among these, some of the promising sources for recovering energy are periodic vibrations generated by rotating machinery or engines. Primarily, the selection of the energy harvester as compared to other alternatives such as battery depends on two main factors cost effectiveness and reliability. In recent years, several energy harvesting approaches have been proposed using solar, thermoelectric, electromagnetic, piezoelectric, and capacitive schemes which can be simply classified in two categories (i) power harvesting for sensor networks using MEMS/thin/thick film approach, and (ii) power harvesting for electronic devices using bulk approach.
Promising applications for piezoelectric energy harvesting have inherent forms of energy to capture, store and use. Examples include “active” sports equipment such as tennis racquets and skis that use strain to power actuators for feedback control loops, and watches that use body motion to supply power. Other applications which have been suggested include the use of aircraft engine vibrations, airflow over wings, vibrations induced by driving on a road, and periodic vibrations generated by rotating machinery or engines. Primarily, the selection of the energy harvester as compared to other alternatives such as battery depends on three main factors, cost effectiveness, profile and reliability. In an other form, the energy harvester can supplement the other energy alternatives such as battery and prolong their lifetime.
Conversion of mechanical low frequency stress into electrical energy is obtained through the direct piezoelectric effect, using a rectifier and DC-DC converter circuit to store the generated electrical energy. There are three primary steps in power generation: (a) trapping mechanical AC stress from available source, (b) converting the mechanical energy into electrical energy with piezoelectric transducer and (c) processing and storing the generated electrical energy. The mechanical output can be in the form of a burst or continuous signal depending on the cyclic mechanical amplifier assembly. Depending on the frequency and amplitude of the mechanical stress, one can design the required transducer, its dimensions, vibration mode and desired piezoelectric material. The energy generated is proportional to frequency and strain and higher energy can be obtained by operating at the resonance of the system.